In many applications, particularly in the medical field, there is a requirement that a circulated fluid be warmed. For example, in connection with cardiac surgery during extracoporial circulation (ECC), the patient is first cooled in order to slow metabolism and thereafter the circulating blood is warmed to return it to body temperature. As another example, heated intravenous fluids are useful in hyperthermic patients and in trauma patients requiring massive IV resuscitation.
One old technique for warming blood is to pass the blood through tubing coils immersed in a warm water bath. However, this warming method is relatively slow and the warming apparatus quite cumbersome.
Also, microwave heating has been used in connection with the heating of blood and intravenous fluids. Typically, a microwave oven is used to warm the fluid in bulk, e.g., blood in a blood bag. However, it has been found to be extremely difficult to achieve uniform heating of the blood due to non-uniform distribution of microwave energy within the oven and the inability, using microwaves, to heat at sufficient depths in a lossy material such as blood which has a high dielectric constant.
Recently, there has been developed an in-line warming apparatus which can warm blood or other fluid flowing through a conduit situated in a heating cavity using microwave energy delivered to that cavity. This apparatus, described in my U.S. Pat. No. 5,073,167, comprises a waveguide heating cavity having a source of microwave energy coupled thereto. A support element forms, with a tixed length of tubing wound about the element, a disposable cartridge which may be positioned in the heating region of the heating cavity. The characteristics, and placement within the heating cavity, of the cartridge are such that rapid, efficient, uniform heating of the fluid results.
Preferably, the apparatus also includes a non-invasive temperature monitor coupled to the heating cavity for monitoring the temperature of the fluid flowing through the tube non-invasively. Also, controls, including a desired operating temperature selector, are provided for combining signals representative of not only cavity temperature, but also inlet and outlet temperatures to control closely the power level of the microwave energy delivered to the fluid in the heating cavity.
In that in-line warming apparatus, the cartridge comprises a bobbin around which the fluid-carrying tubing is wound forming a coil. The cartridge is positioned in the waveguide structure comprising the microwave heating cavity by providing an opening in the waveguide structure and placing the cartridge in the heating region of that structure such that the coil and fluid flowing therethrough are subjected to the fields produced in the waveguide structure when the apparatus is in operation. The bobbin and the tubing are made of dielectric materials which are relatively transparent to the microwave radiation in the cavity and are, therefore, unaffected by the radiation. On the other hand, the fluid flowing through the tubing is relatively lossy and is therefore heated by the microwave energy. By monitoring the temperature of the fluid in the tubing and controlling the energy in response to that temperature, precise warming of the fluid is achieved.
The present invention concerns a cartridge for use in such in-line warming apparatus which maximizes energy coupling to the fluid being warmed and minimizes energy losses from the apparatus.